β1-integrin cytoplasmic subdomains involved in dominant negative function

The beta 1-integrin cytoplasmic domain consists of a membrane proximal subdomain common to the four known isoforms ("common" region) and a distal subdomain specific for each isoform ("variable" region). To investigate in detail the role of these subdomains in integrin-dependent cellular functions, we used beta 1A and beta 1B isoforms as well as four mutants lacking the entire cytoplasmic domain (beta 1TR), the variable region (beta 1COM), or the common region (beta 1 Delta COM-B and beta 1 Delta COM-A). By expressing these constructs in Chinese hamster ovary and beta 1 integrin-deficient GD25 cells (Wennerberg et Id., J Cell Biol 132, 227-238, 1996), we show that beta 1B, beta 1COM, beta 1 Delta COM-B, and beta 1 Delta COM-A molecules are unable to support efficient cell adhesion to matrix proteins. On exposure to Mn++ ions, however, beta 1B, but none of the mutants, can mediate cell adhesion, indicating specific functional properties of this isoform. Analysis of adhesive functions of transfected cells shows that beta 1B interferes in a dominant negative manner with beta 1A and beta 3/beta 5 integrins in cell spreading, focal adhesion formation, focal adhesion kinase tyrosine phosphorylation, and fibronectin matrix assembly. None of the beta 1 mutants tested shows this property, indicating that the dominant negative effect depends on the specific combination of common and B subdomains, rather than from the absence of the A subdomain in the beta 1B isoform.